In order to preserve the strength, durability and integrity of structural members which perform critical structural tasks, it is recognized as desireable, where possible, to forge each such structural member essentially into its final configuration, with a view toward creating and preserving the forged metal grain structure pattern, properly for such strength, integrity and durability. Many such structural members, however, require elongated configurations which are such that this is not practical. Such configurations, for example, are, overall, cruciform, channel or T-shaped in cross section while varying in shape longitudinally. They must fit closely within confined spaces formed by surrounding structures and consequently must be free from any substantial shape or size deviation such as substantial draft. Draft, a tapering thickness deviation, is a normal consequence of conventional two-die forging operations, required to release such parts from die cavities. A typical such structural member has at least two approximately flat walls of substantial extent respectively lying approximately in two planes disposed at an angle to each other, and the conventional initial forging leaves at least one of these walls oversized with very substantial draft, requires an oversize billet to fully fill the die cavities in the initial forging process itself, incurring a significant waste of material in excess flashing, and thereafter requires substantial machining operations to conform the forged structure to the required final configuration. These machining operations are not only an expensive extra step, but also tend to seriously interrupt the metal grain structure pattern achieved by the forging process, and leave minute score gouges in the most vulnerable directions and areas of such mechanical structural members, thus unfortunately promoting premature failure of the members.
This problem is particularly acute for lightweight metallic structural members employed in the aircraft industry where large numbers of such structural members are used as splice fittings to provide attachment links joining adjacent structural frame members of the aircraft.
In the aircraft industry, these structural linking members are variously referred to as splice fittings, clips, stringer clips or cleats. They are used, for example, to attach the ribs of aircraft wings to the stringers of the wings usually by rivoting the walls of the splice fitting respectively to such adjacent structural air-frame members, one of which typically passes above or below the other and at various angles. Regardless of external appearance, the structural integrity of the conventional aircraft depends upon the integrity of such splice fittings. Much of the stress on the frame of the aircraft is in fact transmitted to and through such splice fittings; hence, they perform a critical structural function, essential to safety.
It is the principal object of the present invention to provide a radial hot forging apparatus and method in which high strength structural members having extensive walls may be forged, substantially in finished form in a single operation from an elongated billet of lightweight metal alloy or lightweight metal matrix composite material, without resort to subsequent substantial machining to finish the structure to dimensional tolerances, thereby to preserve the metal grain pattern established by the metal forging process, and running approximately parallel to the outward extent of the walls from the billet axis.
Another object of the invention is to radially forge such structural member by gradually converging together a plurality of at least three pre-heated forging die segments along different approximately radial paths convergent upon a central forging axis to hot forge an elongated metallic billet centered on such axis into said structural member, efficiently and effectively.
A further object of the invention is to achieve said forged structural member, in the form of a splice fitting of lightweight metal alloy or lightweight metal matrix composite material, by the forging process of the invention, wherein said two approximately flat walls thereof, in their forged configuration are of about equal and substantially of uniform thickness throughout almost their entire extent, and have principal metal grain structure running approximately parallel to the extent of said walls, as opposed to transversely thereto along the thickness of said walls, with no substantial machining of said walls incurred to achieve such configuration.
So far as is known the radial forging apparatus and method as herein presented is rovel and unparalleled in the metal forging industry, and in that regard the resulting forged splice fitting for aircraft is unique, as is the radial forging apparatus and process.
Conventional forging apparatus and methods are described in Forging Handbook, edited by T. G. Byrer, sanctioned by the Forging Industries Association and the American Society for Metals, Library of Congress Catalog Number 85-071789, (1985).
There are prior patent references to the use of wedges for split ring forging dies which accommodate a punch die being axially interposed by a forging press to axially forge a billet into the cavity together defined by the two mating dies comprising the split ring. However, such split ring dies are merely locked together by the wedges and the forging forces are applied by the forging press to the axial punch die. (See, for example, Gardener, et. al., U.S. Pat. No. 4,466,266, and Delio, et. al., U.S. Pat. No. 4,559,804). Of course, the split ring dies are used so that they can be taken apart to remove the forged part after the wedge locking mechanism has been disengaged. Also, a pair of clamps moved together by wedges have been used to clamp onto a tube in order to hold the tube for subsequent longitudinal movement of the clamps to upset a portion of the tube wall to form or crimp a flange thereon in a cavity formed by the clamps. (See, for example, Redman, U.S. Pat. No. 4,757,703). So far as understood, these prior patent disclosures are typical of the state of the art in regard to the employment of wedges, and do not address or offer any solution to the metal forging challenges to which the present invention is directed.